1. Field of the Invention
This invention relates to a sheet identifying apparatus and sheet identifying method which identify the type and truth/false of each sheet such as a paper sheet by comparing an image pattern acquired from the sheet with preset reference patterns in an automatic checking apparatus for sheets such as securities.
2. Description of the Related Art
Generally, an identification process for a digital image pattern (which is hereinafter simply referred to as a pattern) tends to require a sensor input of higher resolution as it is desired to perform a higher-degree identification process. However, at this time, it is required to realize the process on the real-time basis with lower calculation cost from the viewpoint of practicality.
Therefore, various methods of selecting and processing part of the pattern (pixels or sets of pixels) are proposed. Generally, since the pattern contains much redundancy, a sufficiently high identification performance can be attained by adequately selecting portions of the pattern and performing a small amount of calculations with the resolution kept high.
For example, the technique for narrowing down to-be-processed pixels by performing a random pixel selecting operation and attaining both of the high-degree resolution and high-speed processes is known (for example, refer to Jpn. Pat. Appln. KOKAI Publication No. H9-134464). With the above well-known technique, since the pixel selecting operation is randomly performed for each checking process, it becomes practically impossible to detect a checking portion thereby to enhance the reliability.
- - - . However, with the above well-known technique, positional weighting is not taken into consideration and it is considered desirable to uniformly distribute the selected pixels in the entire pattern area. Certainly, in order to make it difficult to detect the checking portion, it is idealistic to uniformly select the pixels from the entire pattern area. However, generally, a method of uniformly processing the entire pattern area to attain an identification result has a problem that a local feature cannot be sufficiently distinctly identified or a local variation in the medium (stain, light brush-stroke or the like) gives an influence on the identification performance.
Therefore, it is desired to develop an identification method which can sufficiently identify a local feature while maintaining the high-speed operation and high reliability.
Further, when part of the pattern is selected and processed, an influence is given by a fluctuation in the printing concentration, skewing and sliding of the sheet caused by the feeding state of the sheet in some cases.